DNA fragments from the genome of the nematode Caenorhabditis elegans will be cloned and propagated by splicing into a lambda bacteriophage vector. These vector-borne nematode fragments will be used to detect programmed rearrangements in the corresponddng segments of the genome of developing larvae. A new protocol for detecting rearrangements will be developed for this purpose; this protocol makes no presupposition about the size and topology of the postulated rearrangements, and may be sensitive and rapid enough to survey as much as 10% of the nematode genome for rearrangements occurring in as few as one of the larva's 600 cells. The vector-borne nematode fragments will also be used to detect cells expressing RNA gene-products complementary to the fragments in serially sectioned larvae. The cellular distribution of gene-products will be compared to the distribution of cells expressed genes. Clonally expressed genes may play a key role in the programming of cell fates during development, and DNA rearrangements have received widespread attention as a possible mechanism for programming the expression of clonally-expressed genes.